Datasheet
LTC4085
19
4085fd
–
+
–
+
R
NOM
10k
R
NTC
10k
NTC
V
NTC
9
0.1V
NTC_ENABLE
4085 F04a
LTC4085
TOO_COLD
TOO_HOT
0.74 • V
NTC
0.29 • V
NTC
–
+
8
–
+
–
+
R
NOM
121k
R
NTC
100k
R1
13.3k
NTC
V
NTC
9
0.1V
NTC_ENABLE
4085 F04b
TOO_COLD
TOO_HOT
0.74 • V
NTC
0.29 • V
NTC
–
+
8
LTC4085
(4a) (4b)
Figure 4. NTC Circuits
Thermistors
The LTC4085 NTC trip points were designed to work
with thermistors whose resistance-temperature charac-
teristics follow Vishay Dale’s “R-T Curve 2”. The Vishay
NTHS0603N02N1002J is an example of such a thermis-
tor. However, Vishay Dale has many thermistor products
that follow the “R-T Curve 2” characteristic in a variety of
sizes. Furthermore, any thermistor whose ratio of R
COLD
to R
HOT
is about 7.0 will also work (Vishay Dale R-T Curve
2 shows a ratio of R
COLD
to R
HOT
of 2.815/0.4086 = 6.89).
Power conscious designs may want to use thermistors
whose room temperature value is greater than 10k. Vishay
Dale has a number of values of thermistor from 10k to
100k that follow the “R-T Curve 2”. Using these as indi-
cated in the NTC Thermistor section will give temperature
trip points of approximately 3°C and 47°C, a delta of
44°C. This delta in temperature can be moved in either
direction by changing the value of R
NOM
with respect to
R
NTC
. Increasing R
NOM
will move both trip points to lower
temperatures. Likewise a decrease in R
NOM
with respect
to R
NTC
will move the trip points to higher temperatures.
To calculate R
NOM
for a shift to lower temperature for
example, use the following equation:
R
NOM
=
R
COLD
2.815
•R
NTC
at 25°C
where R
COLD
is the resistance ratio of R
NTC
at the desired
cold temperature trip point. If you want to shift the trip
points to higher temperatures use the following equation:
R
NOM
=
R
HOT
0.4086
•R
NTC
at 25°C
where R
HOT
is the resistance ratio of R
NTC
at the desired
hot temperature trip point.
Here is an example using a 100k R-T Curve 1 thermistor
from Vishay Dale. The difference between the trip points
is 44°C, from before, and we want the cold trip point to
be 0°C, which would put the hot trip point at 44°C. The
R
NOM
needed is calculated as follows:
R
NOM
=
R
COLD
2.815
•R
NTC
at 25°C
=
3.266
2.815
•100k = 116k
APPLICATIONS INFORMATION